Process for producing terephthalic acid

ABSTRACT

Methods of producing terephthalic acid are described. The methods involve using a p-xylene stream enriched with p-toluic acid and optionally terephthalic acid at impurity levels. The p-xylene stream enriched with p-toluic acid, a solvent comprising an ionic liquid and optionally a carboxylic acid, a bromine source, a catalyst, and an oxidizing agent are contacted to produce a product comprising terephthalic acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 13/340,122, filed Dec. 29, 2011, the contents of which are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to terephthalic acid compositions and processes for producing terephthalic acid from a feedstock comprising para-xylene. More particularly, the invention relates to the oxidation of a p-xylene stream enriched with p-toluic acid to produce terephthalic acid.

BACKGROUND OF THE INVENTION

Oxidation of alkyl aromatic compounds, e.g., toluene and xylenes, are important commercial processes. A variety of oxidation products may be obtained including aromatic carboxylic acids such as terephthalic acid (1,4-benzenedicarboxylic acid) and isophthalic acid (1,3-benzenedicarboxylic acid) which are used, for example, in the polymer industry.

It is known that oxidation products, such as aromatic alcohols, aromatic aldehydes, aromatic ketones, and aromatic carboxylic acids, may solidify or crystallize at oxidation conditions and/or as the reaction mixture cools. Thus, mixtures of oxidation products may be produced which require further processing to increase the purity of the desired product. For example, in the production of terephthalic acid, the oxidation product is often referred to as crude terephthalic acid because it contains impurities including color bodies and intermediate oxidation products, especially 4-carboxybenzaldehyde (4-CBA). To obtain polymer grade or purified terephthalic acid, various purification steps are known in the art including: washing the crude terephthalic acid with water and/or a solvent, additional oxidation or crystallization steps, and reacting a solution of dissolved crude terephthalic acid with hydrogen at hydrogenation conditions usually including a catalyst comprising palladium and carbon. Often several purification steps are used.

U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromatic compounds to the corresponding aromatic carboxylic acids. A process for the liquid phase oxidation of alkyl aromatic compounds uses molecular oxygen, a metal or metal ions, and bromine or bromide ions in the presence of an acid. The metals may include cobalt and/or manganese. Exemplary acids are lower aliphatic mono carboxylic acids containing 1 to 8 carbon atoms, especially acetic acid.

U.S. Pat. No. 6,355,835 discloses a process for the preparation of benzene dicarboxylic acids by liquid phase oxidation of xylene isomers using oxygen or air by oxidizing in the presence of acetic acid as a solvent, a cobalt salt as a catalyst, and an initiator. The oxidation step is followed by flashing the reaction mixture to remove volatile substances and cooling and filtering the material to get crude benzene di-carboxylic acid as a solid product and a filtrate. Recrystallizing the crude benzene di-carboxylic acid to obtain at least 99% purity and recycling of the filtrate are also disclosed.

U.S. Pat. No. 7,094,925 discloses a process for preparing an alkyl-aromatic compound. The process includes mixing an oxidizing agent or sulfur compound in the presence of an ionic liquid. Air, dioxygen, peroxide, superoxide, or any other form of active oxygen, nitrite, nitrate, and nitric acid or other oxides or oxyhalides of nitrogen (hydrate or anhydrous) can be used as the oxidizing agent. The process is typically carried out under Bronstead acidic conditions. The oxidation is preferably performed in an ionic liquid containing an acid promoter, such as methanesulfonic acid. The product is preferably a carboxylic acid or ketone or intermediate compound in the oxidation, such as an aldehyde, or alcohol.

U.S. Pat. No. 7,985,875 describes a process for preparing an aromatic polycarboxylic acid by liquid phase oxidation of a di- or tri-substituted benzene or naphthalene compound. The process involves contacting the aromatic compound with an oxidant in the presence of a carboxylic acid solvent, a metal catalyst, and a promoter in a reaction zone. The promoter is an ionic liquid comprising an organic cation and a bromide or iodide anion. The promoter is used in a concentration range of about 10 to about 50,000 ppm (based on solvent) with a preferred range of 10-1,000 ppm. No other promoters, such as bromine-containing compounds, need to be used in the process. The process produces crude terephthalic acid (CTA) having 1.4-2.2% 4-CBA. Purification of the CTA is required to obtain purified terephthalic acid (PTA).

US 2010/0174111 describes a process for purifying aryl carboxylic acids, such as terephthalic acid. The impure acid is dissolved or dispersed in an ionic liquid. A non-solvent (defined as a molecular solvent for which the ionic solvent has high solubility and for which the aryl carboxylic acid has little or no solubility) is added to the solution to precipitate the purified acid.

U.S. Pat. No. 7,692,036, 2007/0155985, 2007/0208193, and 2010/0200804 disclose a process and apparatus for carrying out the liquid-phase oxidation of an oxidizable compound. The liquid phase oxidation is carried out in a bubble column reactor that provides for a highly efficient reaction at relatively low temperatures. When the oxidized compound is para-xylene, the product from the oxidation reaction is CTA which must be purified. Purification is said to be easier than for conventional high temperature processes.

U.S. Publication 2005/0107630 discloses a process for oxidation with oxygen of at least one aromatic hydrocarbon having oxidizable, substituents that maximizes the utilization of oxygen without reduction in the quality of the carboxylic acid products produced by means of a stagewise (two-stage) countercurrent oxidation system. In the first oxidation stage, a product is produced including from about 5.0 to about 85.0 weight percent of the terephthalic acid, from about 2.0 to about 20.0 weight percent of 4-carboxybenzaldehyde, from about 0.0 to about 3.0 weight percent of hydroxymethylbenzoic acid, from about 5.0 to about 65.0 weight percent of p-toluic acid, from about 0.0 to about 30.0 weight percent of p-tolualdehyde, and from about 0.0 to about 35.0 weight percent of p-xylene. This product mix is fed to the second oxidation stage. The product of the first oxidation stage necessarily includes at least about 5.0 weight percent terephthalic acid due to the reaction equilibrium within the first oxidation stage.

SUMMARY OF THE INVENTION

The invention provides a process for the oxidation of para-xylene to terephthalic acid. It has been discovered that the invention may be used to produce terephthalic acid compositions having different amounts of contaminants relative to those observed in conventional processes. In some embodiments, the terephthalic acid compositions produced by the process have low levels of impurities and thus are less costly to purify. In some embodiments, the invention produces polymer grade or purified terephthalic acid without the use of catalytic hydrogenation.

One aspect of the invention is a process for producing terephthalic acid. In one embodiment, the process includes providing a p-xylene stream enriched with p-toluic acid, wherein the p-xylene stream may optionally include terephthalic acid at impurity levels of 4.5 weight percent or less; and contacting the p-xylene stream enriched with p-toluic acid, a solvent comprising an ionic liquid, a bromine source, a catalyst, and an oxidizing agent to produce a product comprising terephthalic acid.

In another embodiment, the process includes providing a p-xylene stream comprising p-toluic acid, wherein the p-xylene stream is produced by a process in which the p-toluic acid is maintained at a level of at least about 5% by weight of the p-xylene stream produced therefrom, and wherein the p-xylene stream may optionally include terephthalic acid at impurity levels of 4.5 weight percent or less; contacting a solvent comprising an ionic liquid, a bromine source, a catalyst, the p-xylene stream of said providing step, and an oxidizing agent, to produce a product comprising terephthalic acid.

In another embodiment, the process includes contacting p-xylene, a first solvent, a first bromine source, a first catalyst, and a first oxidizing agent to produce a p-xylene stream enriched with p-toluic acid, and which may optionally include terephthalic acid at impurity levels of 4.5 weight percent or less; maintaining the p-toluic acid at a level of at least about 5% by weight of the p-xylene stream enriched with p-toluic acid; and contacting the p-xylene stream enriched with p-toluic acid, a second solvent comprising an ionic liquid, a second bromine source, a second catalyst, and a second oxidizing agent to produce a product comprising terephthalic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reaction scheme for the oxidation of p-xylene.

FIG. 2 is a graph showing the effect of starting material on levels of impurities.

DETAILED DESCRIPTION OF THE INVENTION

In general, the invention relates to terephthalic acid compositions and processes for the oxidation of para-xylene to terephthalic acid. In broad terms, the invention is a process for producing terephthalic acid from a p-xylene stream enriched with p-toluic acid.

As discussed below, it was discovered that the production of 4-CBA is significantly lower if the starting material is p-toluic acid rather than p-xylene. As a result, it was determined that starting with a p-xylene feed stream enriched with p-toluic acid would be desirable.

In addition, the general reaction scheme for the production of terephthalic acid through liquid-phase oxidation of p-xylene is shown in FIG. 1. In that series of reactions, the production of p-toluic acid from p-xylene is faster in acetic acid than it is in ionic liquid. However, the subsequent reactions are faster and more selective in an ionic liquid/acetic acid mixture than in acetic acid alone.

The p-xylene stream enriched with p-toluic acid can be obtained in any suitable manner, including but not limited to, oxidation, alkylation and the like. For example, a stream having the required p-toluic acid content could be generated as a product stream in a petroleum or chemical processing complex. If the initial product stream does not have the needed purity, it can be purified to the desired level using known methods.

The p-xylene stream enriched with p-toluic acid contains at least about 5% and less than 90% by weight p-toluic acid. It typically contains at least about 10% by weight, or at least about 15% by wt, or at least about 20% by weight, or at least about 25% by wt, or at least about 30% by weight, or at least about 35% by wt, or at least about 40% by weight, or at least about 45% by wt, or at least about 50% by weight, or at least about 55% by wt, or at least about 60% by wt, or at least about 65% by weight, or at least about 70% by wt, or at least about 75% by wt. It typically contains less than about 85% wt, or less than about 80% wt. The p-xylene stream may also (optionally) include terephthalic acid at impurity levels, that may be introduced into the stream due to the use of impure or “crude” p-toluic acid to enrich the p-xylene stream. As used herein, the term “impurity levels” denotes an amount of terephthalic acid of less than about 4.5 weight percent, for example less than 3.0 weight percent, such as less than 1.5 weight percent, or less than 0.5 weight percent. In some embodiments, impurity levels of terephthalic acid in the p-xylene stream may be as low as 10 parts per million (by weight).

The p-toluic acid level is maintained in solution, as colloids, or as a slurry.

The basic step of the process involves contacting the p-xylene stream enriched with p-toluic acid (however obtained), a solvent comprising an ionic liquid, a bromine source, a catalyst, and an oxidizing agent to produce a product comprising terephthalic acid.

The contacting step(s) may be practiced in laboratory scale experiments through full scale commercial operations. The process may be operated in batch, continuous, or semi-continuous mode. The contacting step can take place in various ways. The order of addition of the components (e.g. p-xylene stream enriched with p-toluic acid, solvent, bromine source, catalyst, and oxidizing agent) is not critical. For example, the components can be added individually, or two or more components may be combined or mixed before being combined or mixed with other components.

The solvent comprises at least one ionic liquid. Two or more ionic liquids can be used, if desired.

Generally, ionic liquids are non-aqueous, organic salts composed of ions where the positive ion is charge balanced with a negative ion. These materials have low melting points, often below 100° C., undetectable vapor pressure, and good chemical and thermal stability. The cationic charge of the salt is localized over hetero atoms, and the anions may be any inorganic, organic, or organometallic species.

Most ionic liquids are formed from cations that do not contain acidic protons. The synthesis of ionic liquids can generally be split into two parts: formation of the desired cation, and anion exchange to form the desired product. Quaternization of an amine or phosphine, for example, is the initial step in the synthesis of the cation of an ionic liquid. If it is not possible to form the desired anion directly by the quaternization reaction, a further step is required.

There are estimated to be hundreds of thousands of simple ion combinations to make ionic liquids, and an almost endless (10¹⁸) number of potential ionic liquid mixtures. This implies that it should be possible to design an ionic liquid with the desired properties to suit a particular application by selecting anions, cations, and mixture concentrations. Ionic liquids can be adjusted or tuned to provide a specific melting point, viscosity, density, hydrophobicity, miscibility, etc. for specific applications. The thermodynamics and reaction kinetics of processes carried out in ionic liquids are different from those in conventional media. This creates new opportunities for catalytic reactions, separations, combined reaction/separation processes, heat transfer agents, hydraulic fluids, paint additives, electrochemistry applications, as well as many others. Ionic liquids do not emit volatile organic compounds (VOCs), providing a basis for clean manufacturing, e.g., “green chemistry.”

The organic cation can comprise a linear, branched, or cyclic heteroalkyl unit, as described in US 2010/0174111. The term “heteroalkyl” refers to a cation comprising one or more heteroatoms chosen from nitrogen, oxygen, sulfur, boron, arsenic, boron, antimony, aluminum, or phosphorous capable of forming a cation. The heteroatom can be a part of a ring formed with one or more other heteroatoms, for example, pyridinyl, imidazolinyl rings, that can have substituted or unsubstituted linear or branched alkyl units attached thereto. In addition, the cation can be a single heteroatom wherein a sufficient number of substituted or unsubstituted linear or branched alkyl units are attached to the heteroatom such that a cation is formed.

Non-limiting examples of heterocyclic and heteroaryl units that can be alkylated to form cationic units include imidazole, pyrazoles, thiazoles, isothiazoles, azathiozoles, oxothiazoles, oxazines, oxazolines, oxazaboroles, dithiozoles, triazoles, selenozoles, oxaphospholes, pyrroles, boroles, furans, thiphenes, phospholes, pentazoles, indoles, indolines, oxazoles, isothirazoles, tetrazoles, benzofuran, dibenzofurans, benzothiophenes, dibenzothoiphenes, thiadiazoles, pyrdines, pyrimidines, pyrazines, pyridazines, piperazines, piperidines, morpholines, pyrans, annolines, phthalazines, quinazolines, and quinoxalines.

The anionic portion of the ionic liquid can comprise an inorganic, organic, or organometallic moiety, as described in US 2010/0174111. Non-limiting examples of anions include inorganic anions: halogens, (e.g., F, Cl, Br, and I); borides, BX₄, wherein X represents halogen, (e.g., BF₄, BCl₄), and the like; phosphates(V), PX₆; PF₆, and the like; arsenate(V), AsX₆; AsF₆, and the like; stibate(V) (antimony), SbX₆; SbF₆, and the like; CO₃ ²⁻; NO₂ ¹⁻, NO₃ ¹⁻, SO₄ ²⁻, PO₄ ³⁻, (CF₃)SO₃ ¹⁻ and their derivatives.

Other non-limiting examples of ionic liquid anions include substituted azolates, that is, five membered heterocyclic aromatic rings that have nitrogen atoms in either positions 1 and 3 (imidazolates); 1, 2, and 3 (1,2,3-triazolates); or 1, 2, 4 (1,2,4-triazolate). Substitutions to the ring occur at positions that are not located in nitrogen positions (these are carbon positions) and include CN (cyano-), NO₂ (nitro-), and NH₂ (amino) group appended to the heterocyclic azolate core.

Further non-limiting examples of anions include substituted or unsubstituted borides: B(R)₄; substituted or unsubstituted sulfates: (RO)S(═O)₂O; substituted or unsubstituted acyl units RCO₂, for example, acetate CH₃CO₂, proprionate, CH₃CH₂CO₂, butyrate CH₃CH₂CH₂CO₂, and benzylate, C₆H₅CO₂; substituted or unsubstituted phosphates: (RO)₂P(═O)O; substituted or unsubstituted carboxylates: (RO)C(═O)O; substituted or unsubstituted azolates wherein the azolate can be substituted on a carbon atom by a unit chosen from cyano, nitro, and amino R can be an organic, inorganic, or organometallic group. Non-limiting examples of R include hydrogen; substituted or unsubstituted linear branched, and cyclic alkyl; substituted or unsubstituted linear, branched, and cyclic alkoxy; substituted or unsubstituted aryl; substituted or unsubstituted aryloxy; substituted or unsubstituted heterocyclic; substituted or unsubstituted heteroaryl; acyl; silyl; boryl; phosphino; amino; thio; and seleno.

In an embodiment, ionic liquids suitable for use include, but are not limited to, one or more of imidazolium ionic liquids, pyridinium ionic liquids, tetra alkyl ammonium ionic liquids, and phosphonium ionic liquids. More than one ionic liquid may be used. Imidazolium, pyridinium, and ammonium ionic liquids have a cation comprising at least one nitrogen atom. Phosphonium ionic liquids have a cation comprising at least one phosphorus atom. In an embodiment, the ionic liquid comprises a cation selected from alkyl imidazolium, di-alkyl imidazolium, and combinations thereof. In another embodiment, the ionic liquid comprises an anion selected from of halides, acetate, carboxylates, and combinations thereof. The ionic liquid may comprise at least one of 1-butyl 3-methyl imidazolium acetate (BMImOAc), 1-butyl 3-methyl imidazolium bromide (BMImBr), 1-hexyl 3-methyl imidazolium acetate, and 1-hexyl 3-methyl imidazolium bromide.

The ionic liquid can be provided, or it can be generated in situ from appropriate precursors, or both. If it is generated in situ, the solvent comprises precursors of one or more ionic liquids. The ionic liquid precursors comprise a cation precursor, such as an alkyl imidazole, alkyl pyridine, alkyl amine, alkyl phosphine, and the like, and an anion precursor, such as alkyl or aryl halides or acetates. In an embodiment, the precursors are methyl imidazole and butyl bromide.

The mode of introducing the ionic liquid precursors may vary depending on the nature of the alkyl aromatics being oxidized and the nature and purity of the product desired. In one mode of addition, the cation precursors and the anion precursors (generally liquids at room temperature and pressure) are mixed with a carboxylic acid (for example, acetic acid) solvent and introduced into the oxidation reactor(s). In another mode of addition, the ionic liquid precursors may be mixed with the alkyl aromatic feed and introduced into the oxidation reactors. In another mode of addition, both cation and anion ionic liquid precursor components may be introduced into the bottom of the reactor without pre-mixing with any other oxidation reactor components such as the feed, carboxylic acid solvent, and catalyst package.

The solvent can also comprise a carboxylic acid. When carboxylic acids are used in the solvent, the amount of carboxylic acid is decreased compared with conventional processes in order to avoid excessive solvent volumes. The carboxylic acid desirably has from 1 to 7 carbon atoms. In an embodiment, the carboxylic acid comprises acetic acid. The solvent may contain more than one carboxylic acid. For example, the solvent may further comprise benzoic acid. In another embodiment, the carboxylic acid of the solvent is acetic acid.

In an embodiment, the solvent has a ratio of the carboxylic acid to the ionic liquid within a range of about 1:16 to 16:1 by weight, or about 1:9 to 9:1 by weight, or about 3:17 to 17:3 by weight, or about 1:4 to 4:1 by weight, or about 1:3 to 3:1 by weight, or about 3:7 to 7:3 by weight, or about 7:13 to 13:7 by weight, or about 2:3 to 3:2 by weight, or about 9:11 to 11:9 by weight, or about 1:1 by weight. In an embodiment, the solvent contains more than 5% by weight ionic liquid, or at least about 6% by weight ionic liquid, or at least about 10% by weight ionic liquid, or at least about 15% by weight ionic liquid, or at least about 20% by weight ionic liquid, or at least about 25% by weight ionic liquid, or at least about 30% by weight ionic liquid, or at least about 35% by weight ionic liquid, or at least about 40% by weight ionic liquid, or at least about 45% by weight ionic liquid. The amount of ionic liquid includes ionic liquid precursors, if present. The optional ionic solid or material capable of forming an ionic salt in solution discussed below, if present, is included in the amount of ionic liquid.

Optionally, an ionic solid, such as ammonium acetate (NH₄OAc) and/or ammonium bromide (NH₄Br), can be added to the mixture. Alternatively, a material which is capable of forming an ionic salt in solution can be added. The material can form the ionic salt in solution by combining with ions present in the solution. For example, in a solution containing bromide (for example in the form of HBr) or acetate ions (for example, in the form of acetic acid), ammonia could combine with the bromide or acetate ions forming ammonium bromide or ammonium acetate. The use of one or more ionic solids or materials capable of forming an ionic salt in solution provided an additional reduction in the level of impurities.

In an embodiment, the amount of ionic solid and material capable of forming an ionic salt in solution ranges from about 5 wt % to about 45 wt %, relative to the weight of the solvent, or from about 10 wt % to about 45 wt %, relative to the weight of the solvent. The solvent includes the carboxylic acid, the ionic liquid and/or ionic liquid precursors, the optional ionic solid or material capable of forming an ionic salt in solution, and the optional water.

Optionally, the solvent may further comprise water. The water may be added to the mixture or generated in the mixture during the oxidation process. In an embodiment, the amount of water ranges from about 0.01 wt % to about 5 wt %, relative to the weight of the carboxylic acid. The amount of water may range from about 0.1 wt % to about 2 wt %, relative to the weight of the carboxylic acid.

In an embodiment, the ratio of solvent to p-xylene stream enriched with p-toluic acid (and possibly terephthalic acid at impurity levels of less than about 4.5 weight percent) in the mixture ranges from about 1:1 to about 10:1 by weight, or from about 1.5:1 to about 6:1 by weight, or from about 2:1 to about 4:1 by weight. The solvent includes the carboxylic acid, the ionic liquid and/or ionic liquid precursors, the optional ionic solid or material capable of forming an ionic salt in solution, and the optional water.

The catalyst comprises at least one of cobalt, manganese, titanium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium and zirconium. In an embodiment, the catalyst comprises cobalt and manganese. The metal may be in the form of an inorganic or organic salt. For example, the metal catalyst may be in the form of a carboxylic acid salt, such as, a metal acetate and hydrates thereof. Exemplary catalysts include cobalt (II) acetate tetrahydrate and manganese (II) acetate, individually or in combination. In an embodiment, the amount of manganese (II) acetate is less than the amount of cobalt (II) acetate tetrahydrate by weight.

The amount of catalyst used in the invention may vary widely. For example, the amount of cobalt may range from about 0.001 wt % to about 2 wt % relative to the weight of the solvent. In an embodiment, the amount of cobalt ranges from about 0.05 wt % to about 2 wt % relative to the weight of the solvent. The amount of manganese may range from about 0.001 wt % to about 2 wt % relative to the weight of the solvent. In an embodiment, the amount of manganese ranges from about 0.05 wt % to about 2 wt % relative to the weight of the solvent. In another embodiment, the ratio of cobalt to manganese ranges from about 3:1 to about 1:2 by weight on an elemental metal basis.

Bromine sources are generally recognized in the art as being catalyst promoters and include bromine, ionic bromine, e.g. HBr, NaBr, KBr, NH₄Br; and/or organic bromides which are known to provide bromide ions at the oxidation conditions, such as, benzylbromide, mono and di-bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylene di-bromide. In an embodiment, the bromine source comprises or consists essentially of or consists of hydrogen bromide. The amount of hydrogen bromide may range from about 0.01 wt % to about 5 wt %, relative to the weight of the solvent. In another embodiment, the amount of hydrogen bromide ranges from about 0.05 wt % to about 2 wt %, relative to the weight of the solvent. The solvent includes the carboxylic acid, the ionic liquid and/or ionic liquid precursor, the optional ionic solid or material capable of forming an ionic salt in solution, and the optional water.

Suitable oxidizing agents for the process provide a source of oxygen atoms to oxidize the p-xylene and/or p-toluic acid, and/or another intermediate oxidization product at the oxidation conditions employed. Examples of oxidizing agents include peroxides, superoxides, and nitrogen compounds containing oxygen such as nitric acids. In an embodiment, the oxidizing agent is a gas comprising oxygen, e.g. air, carbon dioxide, and molecular oxygen. The gas may be a mixture of gases. The amount of oxygen used in the process is preferably in excess of the stoichiometric amount required for the desired oxidation process. In an embodiment, the amount of oxygen contacted with the mixture ranges from about 1.2 times the stoichiometric amount to about 100 times the stoichiometric amount. Optionally, the amount of oxygen contacted with the mixture may range from about 2 times the stoichiometric amount to about 30 times the stoichiometric amount.

At least a portion of the components provides a liquid phase, although dissolution of one or more of the mixture components may not be complete at any or some time during the process. The liquid phase may be formed by mixing the components at ambient conditions. In another embodiment, the liquid phase is formed as the temperature of the mixture increases to the oxidation temperature. A mixture of the components may be formed prior to the oxidation step, in the same or different vessel as that used in the oxidation step. In another embodiment, a mixture of the components is formed in an oxidation reactor, e.g. adding various streams of the components individually and/or in combination to a continuous or semi-continuous oxidation reactor. The combined components, and/or various streams of the components may be heated before they are mixed together.

Though many conventional alkyl aromatic oxidation processes are typically conducted in a mixed phase, and often include three phases (e.g. solid, gas, and liquid), they are frequently referred to in the art as “liquid phase” oxidation processes because the oxidation conditions are maintained to provide at least a portion of the mixture in the liquid phase. It is also known in the art that the number of phases present may vary over time during the process. Processes according to the instant invention may also be conducted in a liquid phase or mixed phase in a similar manner as known in the art.

Conventional, liquid phase oxidation reactors as known in the art may be used to practice the invention. Examples include vessels, which may have one or more mechanical agitators, and various bubble column reactors such as those described in U.S. Pat. No. 7,692,036. It is also known to design, operate, and control such reactors and the oxidation reaction for the oxidation conditions employed including, e.g., the temperature, pressure, liquid and gas volumes, and corrosive nature of the liquid and gas phases where applicable. See, e.g. U.S. Pat. No. 7,692,036 and U.S. Pat. No. 6,137,001.

The contacting step[s] can take place under oxidizing conditions, if desired. Suitable oxidizing conditions generally include a temperature ranging from about 125° C. to about 275° C. and a pressure ranging from about atmospheric, i.e. 0 MPa(g), to about 6 MPa(g) and a residence time ranging from about 5 seconds to about 2 weeks. That is, the mixture has a temperature and a pressure within these ranges and may be maintained within these ranges for a period of time within the residence time range. In another embodiment, the temperature ranges from about 175° C. to about 225° C.; and the temperature may range from about 190° C. to about 235° C. In an embodiment, the pressure ranges from about 1.2 MPa(g) to about 6.0 MPa(g); and the pressure may range from about 1.5 MPa(g) to about 6.0 MPa(g). In a further embodiment, the residence time ranges from about 10 minutes to about 12 hours. The oxidation temperature, pressure and residence time may vary based on a variety of factors including for example, the reactor configuration, size, and whether the process is, batch, continuous, or semi-continuous. An oxidation condition may also vary based on other oxidation conditions. For example, use of a particular temperature range may enable use of a different residence time range.

In an embodiment, the terephthalic acid produced by the instant invention may precipitate, crystallize, or solidify in a liquid phase mixture at the oxidation conditions and/or as the mixture cools. Thus, a mixture according to the invention may further comprise solid terephthalic acid. Other compounds, including color bodies, and other oxidation products may solidify with or be trapped in the solid oxidation product thus reducing the purity of the desired product. In an embodiment, the mixture comprises a liquid phase. The mixture may comprise a gas phase such as when the oxidizing agent is added as a gas. The mixture may comprise a solid phase e.g. a mixture component, an oxidation product, or a by-product fails to dissolve or solidifies in the mixture. In an embodiment, the mixture comprises a liquid phase, a solid phase and optionally a gas phase. In another embodiment, the mixture comprises a liquid phase and a gas phase.

As noted above and discussed below, it has been discovered that the invention may be used to produce an oxidation product having different amounts of contaminants relative to those observed in conventional processes. In addition, the invention provides new ways to control the level of various contaminants in the oxidation product. In an embodiment, a process according to the invention further comprises forming the oxidation product as a solid, optionally at the oxidizing conditions, to produce the solid oxidation product and a mother liquor. The solid oxidation product may be separated from the mother liquor, i.e. liquid phase, and the mother liquor of the process may be recycled and reused in the contacting step or other steps of the process described below.

Processes according to the invention, may comprise one or more additional oxidizing steps. In an embodiment, a second oxidation step includes a second oxidizing temperature that is lower than the temperature of the first oxidizing step. Processes according to the invention may include additional contacting steps of the invention as described herein, and/or the invention may be combined with other oxidizing steps such as conventional oxidizing steps known in the art. Multiple contacting and/or oxidation steps may be conducted in series and/or parallel and may be combined with other process steps such as purification steps described herein.

In another embodiment, the invention further comprises purifying the oxidation product. Purifying may comprise one or more additional steps to isolate and purify the oxidation product. Examples of purifying steps include: separating wherein the oxidation product is separated from the mother liquor or another liquid phase such as by filtration and/or centrifugation; washing wherein the oxidation product is washed, for example with water and/or another solvent component; drying the oxidation product; and hydrogenation processes. Although hydrogenation processes can be used for purification, they are less desirable than other purification methods due to the cost. Such additional processing steps have been described in the general literature and are well known to those of ordinary skill in the art to be used in various combinations to purify oxidation products of the invention. See for example, the references cited in this application and the art cited therein.

A purification step of the instant invention may further comprise one or more solvent contacting steps. A solvent contacting step comprises contacting an oxidation product, also including washed or dried solid oxidation products, with a third solvent comprising at least one of water, a carboxylic acid, an ionic liquid and/or ionic liquid precursor, and a mother liquor to produce a purified oxidation product. In an embodiment, the solvent of the solvent contacting step contains ionic liquid and carboxylic acid, and optionally mother liquor. The composition of the solvent for the solvent contacting step can be as described above for the contacting step.

Solvent contacting may leach impurities from the solid oxidation product, and/or the oxidation product may be partially or completely dissolved in the solvent. Solvent contacting conditions include a solvent contacting temperature. The solvent contacting temperature may be lower than the oxidation temperature. In an embodiment, the solvent contacting temperature is at least 20° C. lower than the oxidation temperature. Solvent contacting may be practiced for example in the one or more crystallizers that follow the oxidation reactor in some conventional processes. The oxidation product may solidify, precipitate, or crystallize in the solvent of the solvent contacting step.

The product made by the process, either initially or following one or more additional oxidizing and/or purification steps, can contain less than about 2500 ppm 4-CBA, or less than about 2000 ppm 4-CBA, or less than about 1500 ppm 4-CBA, or less than about 1000 ppm 4-CBA, or less than about 750 ppm 4-CBA, or less than about 500 ppm 4-CBA, or less than about 250 ppm 4-CBA, or less than about 100 ppm 4-CBA, or less than about 50 ppm 4-CBA, or less than about 25 ppm 4-CBA.

It should be noted that the terms “first”, “second”, and “third” etc. are being used to distinguish one component, or composition, or stage, or zone, or reactor etc. from another. It is not necessarily the case that a “second” stage or zone, for example, physically or temporally follows a “first” stage or zone. Depending on the context, it could be before or after, as would be understood by those of skill in the art.

EXAMPLES

The examples are presented to further illustrate some aspects and benefits of the invention and are not to be considered as limiting the scope of the invention.

Example 1

Experimental procedure: In a fume hood, load a Parr reactor with the specified amounts of components for the given experiment seal the reactor. The Parr reactor includes a gas distributor to disperse the gas through a 1.6 mm opening into the liquid, a mechanical gas entrainment stirrer, and baffles to ensure thorough mixing. Install the Parr reactor in a heater assembly at room temperature and connect a gas supply line to the reactor and a condenser to the reactor outlet. During operation, gases exit the reactor through the condenser then a trap, then a back-pressure regulator. Connect a safety vent having a rupture disk, and thermocouples to the reactor. Connect a cooling water recirculator to the condenser and begin to recirculate cooling water. Pressure test the Parr reactor at room temperature and 1.4 MPa(g) (200 psig) using nitrogen until there is no decrease in pressure for 15 minutes. Set the back pressure regulator on the reactor outlet to the experimental pressure and pressure test the reactor under nitrogen.

Begin raising the reactor temperature to the experimental temperature under the nitrogen atmosphere. Always follow all instructions for the specific reactor including temperature and pressure limits. When the reactor reaches the desired temperature begin adding air at the experimental rate and monitor the reactor temperature and pressure for the duration of the test. During the test, the air flow into the reactor is maintained at 2500 standard cm³ per minute, the pressure is maintained at 4.1 MPa(g), and the stirrer is maintained at 1600 rpm. At the end of the test shut off the heater, cut the air flow and allow the reactor to cool. When the reactor cools to less than about 35° C., open the back pressure valve, stop the cooling water, and remove and empty the reactor to obtain the solid oxidation product and mother liquor.

The mother liquor and products are filtered under vacuum to separate the solids and liquid. The solids are then mixed with approximately 100 cc deionized water at room temperature and decanted. The room temperature deionized water mixing and decanting is repeated two additional times. A fourth wash with deionized water is heated to approximately 95° C. for 30 minutes and then filtered. The solids are dried at 80° C. for 8-24 hours before analyzing.

Examples 2-3

Examples 2-3 were individual tests conducted using the equipment and procedure given in Example 1. The components of the mixture, given in grams, operating temperature, time, and air flow, and results are given in Table 1.

Example 2

p-Xylene used as the starting material under oxidizing conditions.

Example 3

Same oxidizing conditions as Example 2 except p-toluic acid was used as the starting material. The amount of 4-CBA, and p-toluic acid in the product dropped significantly, and the benzoic acid dropped slightly, as shown in FIG. 2. The recovery and the selectivity increased.

Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.

TABLE 1 Example number 2 3 Oxidation Temperature, ° C. 215 215 Oxidation Time 3 3 Air Flow, sccm 2500 2500 Cooling Fast Fast Components (g) p-Xylene 20 p-Toluic Acid 20 Acetic Acid 44 44 BMImOAc 20 20 BMImBr 16 16 NH₄OAc 12 12 HBr 0.4 0.4 H₂O 0.4 0.4 Co(OAc)₂—4H₂O 0.8 0.8 Mn(OAc)₂ 0.6 0.6 Analysis of Product Terephthalic acid (%) 97.2 99.1 4-CBA (ppm) 1081 230 Benzoic acid (ppm) 1163 1143 p-Toluic acid (ppm) 25,607 7183 Visual Product Color White White Recovery (%) 85.2 91.9 

What is claimed is:
 1. A process for producing terephthalic acid comprising: providing a p-xylene stream enriched with p-toluic acid and optionally comprising terephthalic acid impurity levels of less than 4.5 weight percent; introducing the p-xylene stream into an oxidation reactor; and in the oxidation reactor, contacting the p-xylene stream enriched with p-toluic acid, a solvent comprising an ionic liquid, a bromine source, a catalyst, and an oxidizing agent to produce a product comprising terephthalic acid.
 2. The process of claim 1, wherein in the providing step, the p-xylene stream comprises greater than 5% by weight p-toluic acid.
 3. The process of claim 1, wherein the solvent further comprises a carboxylic acid.
 4. The process of claim 1, wherein the solvent has a ratio of the carboxylic acid to the ionic liquid within a range of 1:16 to 16:1 by weight.
 5. The process of claim 1, wherein the contacting step is performed under oxidation conditions.
 6. The process of claim 1, wherein the catalyst comprises at least one of cobalt, titanium, manganese, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.
 7. The process of claim 1 wherein the ionic liquid is formed in situ from at least one ionic liquid precursor wherein the contacting step is performed in a reaction zone.
 8. The process of claim 1 wherein a cation of the ionic liquid is formed from imidazoles, pyrazoles, thiazoles, isothiazoles, azathiozoles, oxothiazoles, oxazines, oxazolines, oxazaboroles, dithiozoles, triazoles, selenozoles, oxahospholes, pyrroles, boroles, furans, thiphenes, phospholes, pentazoles, indoles, indolines, oxazoles, isothirazoles, tetrazoles, benzofuran, dibenzofurans, benzothiophenes, dibenzothoiphenes, thiadiazoles, pyrdines, pyrimidines, pyrazines, pyridazines, piperazines, piperidines, morpholines, pyrans, annolines, phthalazines, quinazolines, quinoxalines, or combinations thereof.
 9. The process of claim 1 wherein an anion of the ionic liquid is halides, borides, phosphates, arsenates, stibates, acetate, carboxylates, azolates, sulfates, acyl units, CO₃ ²⁻; NO₂ ¹⁻, NO₃ ¹⁻, SO₄ ²⁻, PO₄ ³⁻, (CF₃)SO₃ ¹⁻, derivatives thereof, or combinations thereof.
 10. A process for producing terephthalic acid comprising: providing a p-xylene stream comprising p-toluic acid, wherein the p-xylene stream is produced by a process in which the p-toluic acid is maintained at a level of at least 5% by weight of the p-xylene stream produced therefrom, and wherein the p-xylene stream optionally includes terephthalic acid at impurity levels of less than 3.0 weight percent; introducing the p-xylene stream into an oxidation reactor; and in the oxidation reactor, contacting a solvent comprising an ionic liquid, a bromine source, a catalyst, the p-xylene stream of the providing step, and an oxidizing agent, to produce a product comprising terephthalic acid.
 11. The process of claim 10, wherein the solvent further comprises a carboxylic acid.
 12. The process of claim 10, wherein the solvent has a ratio of the carboxylic acid to the ionic liquid within a range of 1:16 to 16:1 by weight.
 13. The process of claim 10, wherein said contacting step is performed under oxidation conditions.
 14. A process for producing terephthalic acid comprising: contacting p-xylene, a first solvent, a first bromine source, a first catalyst, and a first oxidizing agent to produce a p-xylene stream enriched with p-toluic acid and optionally comprising terephthalic acid at impurity levels of less than 1.5 weight percent; maintaining the p-toluic acid at a level of at least 5% by weight of the p-xylene stream enriched with p-toluic acid; contacting the p-xylene stream enriched with p-toluic acid, a second solvent comprising an ionic liquid, a second bromine source, a second catalyst, and a second oxidizing agent to produce a product comprising terephthalic acid.
 15. The process of claim 14, wherein the solvent comprises a carboxylic acid.
 16. The process of claim 14, wherein the contacting steps are done under oxidation conditions.
 17. The process of claim 14, wherein the catalyst comprises at least one of cobalt, titanium, manganese, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.
 18. The process of claim 14, wherein the contacting steps are performed in a reaction zone.
 19. The process of claim 14 wherein contacting p-xylene, a first solvent, a first bromine source, a first catalyst, and a first oxidizing agent is performed in a first reaction zone, and wherein contacting the p-xylene stream enriched with p-toluic acid, a second solvent comprising an ionic liquid, a second bromine source, a second catalyst, and a second oxidizing agent is performed in a second reaction zone.
 20. The process of claim 19 wherein the first reaction zone and the second reaction zone are in the same reactor or in different reactors. 